The preparation of elastic slabstock foams for a very wide range of applications takes place in the low density region using organic, fully halogenated blowing agents such as monofluorotrichlororomethane, which is no longer used for ecological reasons. In the context of this change-over, dichloromethanc (MC) is now frequently used. This blowing agent causes two effects, which is of relevance to foam production. One effect is the lowering of the density as the actual blowing effect and the other effect is the withdrawal of energy from the reacting system during conversion to the gaseous state. As a result, when using large amounts of water, and thus, also isocyanates, it is possible to vary the hardness to a certain extent within the low density region. These types of effects cannot be produced by using water on its own because the exothermal effect produces internal temperatures which cause discolorations in at least the core. A further step in the direction of an ecologically and more soundly based production process is the use of liquid carbon dioxide as a blowing agent, as is described, for example, in DE 4,422,568. Effective operation of the decompression chamber with perforated plate inserts in front of the raw material discharge point is critical for use of this process.
Due to the different energies of evaporation of the blowing agents CO.sub.2 and MC, it is not possible to prepare equivalent types of expanded material in a reliable production process because the internal temperature of the slabs is different at comparable densities.
It has now been found that, by means of specific polyols and with liquid CO.sub.2 as an additional blowing agent, qualities of expanded materials can be produced without problems relating to production engineering or mechanical properties and that these are characterized by high compressive strength and better elasticity.